The present invention concerns a pressure wave machine operating as a pressure exchanger, in particular for use as the high-pressure compressor for gas turbines.
Pressure wave machines operate either as pressure exchangers or as pressure converters. The latter currently find application mainly as pressure wave superchargers for internal combustion engines. In these, the quantity of compressed air Q.sub.A is equal to the quantity exhaust gas Q.sub.G but post-compression is then used to bring the air to a pressure p.sub.3 higher than that of the exhaust gas pressure p.sub.2 before the supercharger.
In pressure exchangers (the first mentioned type of pressure wave machines) on the other hand, a quantity Q.sub.A of cold air is brought from an initial pressure p.sub.1 to a final pressure which is the pressure p.sub.2 of the hot exhaust gases before entry into the pressure exchanger. During the same period of time, the quantity Q.sub.G of the hot exhaust gas is expanded from the initial pressure p.sub.2 to the final pressure p.sub.1. There is, therefore, only an exchange gas pressure and not a compression of the air beyond exhaust gas pressure p.sub.2. The proportion of the enthalpy in the hot exhaust gases (when a pressure exchanger is used in a gas turbine, this would be more appropriately referred to as driving gases) which is not used for additional compression to p.sub.3 --such as occurs in the pressure converter--is used in a pressure exchanger to compress an additional quantity of air which exceeds the air quantity corresponding to the driving gas quantity Q.sub.G. In a pressure exchanger, therefore, the total air quantity Q.sub.A is greater than the driving gas quantity Q.sub.G.
The previously used indices of p.sub.1, p.sub.2 and p.sub.3 indicate the pressure level of the exhaust gas and the air using the designation usual for pressure wave machines. The Index 1 applies to the low pressure side of the air and exhaust gas in both the pressure converter and the pressure exchanger. In the case of the pressure exchanger, the Index 2 applies to the high pressure side of both the exhaust gas and the air whereas, although the Index 2 still applies to the high pressure gas in the case of the pressure converter, the Index 3 applies to the high pressure air, this expressing the fact that this pressure p.sub.3 is greater than p.sub.2 because of the postcompression.
In order to define the location at which the particular pressure is present, further indices U and D are added to the symbols p.sub.1 and p.sub.2, U and D standing for "upstream" and "downstream" of the pressure wave machine. This is not necessary in the case of the highest pressure occurring in a pressure converter, p.sub.3, because it appears alone and is unambiguously defined.
These extra indices U and D therefore refer to the inlet and outlet side for each of the two media air and exhaust gas. In consequence, p.sub.1U is the pressure of the air entering the pressure wave machine (low pressure air), p.sub.1D is the pressure of the expanded exhaust gas at outlet (low pressure gas), p.sub.2U is the exhaust gas before the pressure wave machine and, in the case of the pressure exchanger, p.sub.2D is the air pressure after the pressure wave machine (high pressure gas and high pressure air, respectively). In the case of the pressure converter, the supercharged pressure p.sub.3 appears in place of p.sub.2D. Since, for both pressure exchangers and pressure converters, p.sub.1U is approximately equal to p.sub.1D and p.sub.2U is approximately equal to p.sub.2D in the case of the pressure exchanger whereas, in the case of the pressure converter, p.sub.2U is less than p.sub.3, the two types of pressure wave machines can be characterised by the pressure relationships and the air and exhaust gas quantity relationships as shown below, if the indices U and D are omitted and, for both types, the symbol p.sub.3 is used instead of p.sub.2D :
______________________________________ Pressure converter Pressure exchanger ______________________________________ p.sub.1 &lt; p.sub.2 &lt; p.sub.3 p.sub.1 &lt; p.sub.2 = p.sub.3 Q.sub.G = Q.sub.A Q.sub.G &lt; Q.sub.A ______________________________________
In the case of the pressure converter, the quotient p.sub.3 /p.sub.2 is a measure of the efficiency but in the case of the pressure exchanger, the corresponding quantity is the quotient Q.sub.A /Q.sub.G. The pressure exchanger is a constant pressure, pressure wave machine and, as such, cannot be considered for conventional applications, i.e. as a pressure wave supercharger, because it supplies surplus compressed air. This property, on the other hand, makes it particularly suitable for use as the high pressure compressor in gas turbines. Since, like conventional gas turbine compressors, it normally operates only at a fixed speed and load, i.e. over a fixed temperature range, the pressure wave process can be precisely matched to this operating condition. Under these conditions, there are no wandering waves which have to be captured by the known pockets in the air and gas casings. The scavenging zone and the charging zone in the rotor cells can be designed in a practically ideal manner because only one expansion wave occurs from the exhaust gas opening edge to the air opening edge and only one compression wave occurs from the exhaust gas closing edge to the air closing edge.